Electrolysis cell and method of generating halogen

ABSTRACT

Halogen is produced by electrolyzing an aqueous halide in a specially designed cell. The cell comprises an anolyte chamber and a catholyte chamber separated by a permeable membrane or diaphragm, notably an ion exchange (generally cation exchange) polymer. At least one electrode comprises at least two sections. One section comprises a gas and electrolyte permeable layer, sheet or mat having a catalytic surface, i.e. one having a low overvoltage, (low hydrogen overvoltage if the cathode and low halogen overvoltage if the anode). This layer is spaced from the membrane by a second portion comprising an electroconductive resiliently compressible layer or mat, which is in contact with the membrane on one side thereof, the other side thereof being in contact with the main cathode. 
     This second or spacer section advantageously has an electrode surface having a higher overvoltage than the first electrode surface. Preferably the cathode has the above construction. 
     Upon electrolysis of alkali metal chloride or other halide in such a cell and with a cathode of the type described above, a low voltage is obtained even at high current densities and the cathode efficiency is high.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation in part of copending U.S. applicationSer. No. 268,431, filed May 29, 1981 and entitled "Electrolysis Cell andMethod of Generating Halogen" now U.S. Pat. No. 4,381,979, which in turnis a continuation in part of copending U.S. application Ser. No.212,172, filed Dec. 2, 1980 2nd still pending entitled "ElectrolysisCell and Method of Generating Halogen" which in turn is a contination inpart of U.S. patent application Ser. No. 102,629, filed Dec. 11, 1979entitled "Novel Electrolysis Cell and Method of Electrolyzing Halides"and now U.S. Pat. No. 4,340,452, and is a continuation in part ofcopending U.S. application Ser. No. 382,691 filed May 27, 1982 entitled"Novel Electrolysis Cell and Method of Electrolyzing Halides" now U.S.Pat. No. 4,468,311, which in turn is a division of said U.S. applicationSer. No. 102,629 now U.S. Pat. No. 4,343,690.

DESCRIPTION Background Art

It is known to electrolyze aqueous alkali metal chloride or like halidein a membrane cell having an ion exchange (normally cation exchange)membrane which separates anode from cathode. Since the membrane itselfis generally impermeable or substantially so to gas and liquid flow, theelectrolysis generates chlorine at the anode and alkali at the cathode,the alkali being of high purity and containing only very low chlorideconcentration.

One type of cell which has been proposed for such electrolysis is thesolid polymer electrolyte cell.

A solid polymer electrolyte cell is characterized by an ion exchangemembrane, which separates electrode of the cell and by the fact that oneor preferably both electrodes are in contact with the membrane. Thesolid polymer electrolyte cells present (with respect to conventionalmembrane cells in which the cathode and frequently both anode andcathode are separated from the membrane), several advantages useful indifferent electrolysis processes. More precisely:

(1) The overall voltage between electrodes is lower because theinterelectrodic distance is reduced practically to the membranethickness.

(2) The so-called "bubble effect" is eliminated or at least minimized,i.e. the difficulty normally encountered in electrolytic processes wheregas is evolved at the electrode accumulates in the zone betweenelectrodes is avoided because evolved gas can be released behind theelectrodes to the inside of the cell compartment.

(3) The cell may be very compact and thus the ohmic drops at the currentdistribution structures can be reduced.

The ion permeable diaphragms are cation exchange polymers in the form ofthin flexible sheets or membranes. Generally they are imperforate and donot permit a flow of anolyte into the cathode chamber but it has alsobeen suggested that such membranes may be provided with some smallperforations to permit a small flow of anolyte therethrough, althoughthe bulk of the work appears to have been accomplished with imperforatemembranes.

Typical polymers which may be used for this purpose include fluorocarbonpolymers such as polymers of trifluoroethylene or tetrafluoroethylene orcopolymers thereof which contain ion exchange groups are used for thispurpose. The ion exchange groups normally are cationic groups includingsulfonic acid, sulfonamide, carboxylic acid, phosphoric acid and thelike, which are attached to the fluorocarbon polymer chain throughcarbon and which exchange cations. However, they may also contain anionexchange groups. Typical such membranes have the general formula:##STR1## Such membranes include typically those fluorocarbon ionexchange polymers manufactured by the Du Pont Company under the tradename of "Nafion" and by Asahi Glass Company of Japan under the tradename of "Flemion". Patents which describe such membranes include BritishPat. No. 1,184,321 and U.S. Pat. Nos. 3,282,875 and 4,075,405.

Since these diaphragm are ion permeable but do not permit anolyte flowtherethrough, little or no halide ion migrates through the diaphragms orsuch a material in an alkali chloride cell and therefore the alkali thusproduced contains little or no chloride ion. Furthermore, it is possibleto produce a more concentrated alkali metal hydroxide in which thecatholyte produced may contain from 15 to 45% NaOH by weight or overhigher. Patents which describe such a process include U.S. Pat. Nos.4,111,779 and 4,100,050 and many others. The application of an ionexchange membrane as an ion permeable diaphragm has been proposed forother uses such as in water electrolysis.

In cells of the type contemplated, the cathode is in close proximity toor in direct contact with the ion exchange membrane. They must besufficiently permeable to permit rapid escape of evolved gas from thepoints of their evolution and to provide ready access of liquidelectrolyte to these points as well as rapid removal of evolved alkalior other electrolysis produced from such points. Thus the electrodes arenormally quite porous.

One difficulty which has been encountered with permeable cathodes whichare in direct contact with or bonded to the membrane is that cathodicefficiency is relatively low for example 85% or below and that oxygen inappreciable concentration, for example above 0.5 to 1% or more byvolume, is evolved in the chlorine produced.

Apparently some portion of the alkali metal hydroxyl evolved at thecathode tends to migrate through the membrane. This may be due to thefact that caustic soda produced at the interface is not sufficiently anduniformly diluted by the catholyte within the cathode compartment of thecell.

The high alkalinity may induce dehydration of the membrane withconsequent decrease of the electrical conductivity, moreover the highconcentration gradient increases the back-diffusion of the hydroxyl ionstowards the anode with a resulting loss of the faraday efficiency.

The creation of varying gradients of alkalinity on or in the membranemay cause membrane shrinking and membrane swelling in localized areasand continual changing of these events and this may result in detachmentand/or loss of cathode layer or cathodic material. Whatever the actualmechanism, the adverse results referred to above accrue.

BEST AND VARIOUS MODES OF CARRYING OUT THE INVENTION

According to this invention halogen is effectively generated byelectrolyzing an aqueous halide in an electrolytic cell having a pair ofopposed electrodes separated by an ion permeable separator, preferablyan ion exchange polymer and where at least one electrode, preferably thecathode, has two layers. The first layer is resistant to chemical andelectrochemical attack and has a low overvoltage being readily capableof functioning as an electrode and evolving electrolysis product byelectrolysis. The second such layer has a higher overvoltage (hydrogenovervoltage in the case of the cathode surface or chlorine overvoltagein the case of anode surface) and is between the lower overvoltagesurface and the membrane, generally being in direct contact with themembrane. Of course both surfaces are electroconductive and are capableof being polarized as an electrode. Furthermore both surfaces are indirect electrical contact so that there is little or substantially nopotential difference between them.

Since the first or rear most cathode section has a lower hydrogenovervoltage surface than that of the front section engaging the membranea major portion and even substantially all of the cathodic electrolysisoccurs at points spaced by the spacer or barrier from the membrane asdistinguished from on or close to the membrane surface.

The cathode where the major electrolysis takes place is readily porousand permits ready flow including lateral flow of catholyte therethrough.Thus it may be in the form of fine mesh flexible electroconductive metalscreen having 3 to 10 mesh openings per centimeter or a mat ofundulating wire screen or a combination of these elements. The openingsare relatively large and thus provide channels adjacent to the points ofcontact between the conductive second layer or spacer and the maincatalytic catode section whereby catholyte may flow edgewise along thecatalytic cathode surface and adjacent these points thereby sweepingaway evolved alkali from the front portion of the cathode, as well asfrom the areas remote from the membrane.

For example, the more active cathode layer may have a surface comprisinga platinum group metal or oxide thereof which has a very low hydrogenovervoltage. In that case the intermediate spacer of layer can have anelectroconductive surface of a metal or of an oxide which is higher inovervoltage. A porous silver or stainless steel or nickel screen may beused for this purpose. As will be understood other conductive materialswhich are resitant to corrosion in the alkaline cathode area may also beused. The intermediate section in any case is porous and permeable toelectrolyte. Being quite electroconductive, it may cooperate intransmitting current to the more remote active cathode areas withoutserious increase in overall voltage.

According to the preferred embodiment of the present invention, theintermediate or spacer layer of the multilayered cathode comprises anelectroconductive resiliently compressible wire mat which has a surfaceof higher hydrogen overvoltage than the surface of the main or catalyticcathode layer.

The resiliently compressible wire mat forming the intermediate or spacerlayer of the cathode is advantageously compressed, upon assembly of thecell, between the membrane and the active or catalytic layer of thecathode. Therefore the intermediate resilient layer exerts an elasticreaction force against the membrane and the active layer duringoperation and effectively maintains spaced the surface of the membraneand that of the active cathode. Thus the resiliently compressed wire matforming the intermediate or spacer layer besides acting to maintain acertain separation between the main active layer of the electrode andthe surface of the membrane, also provides for restraining the flexiblemembrane from fluttering under the action of the gas bubbles inducedturbulence of both the anolyte and the catholyte or from bending towardsthe anode or the cathode under the action of varying hydraulic headsdifferentials. This is of great importance since it has been found thatmembranes which were assembled in cells without resilient or other meanscapable to maintain them firmly in place are often subjected to chafingby continuous rubbing of the membrane against the foraminous metalelectrodes.

The rigid mechanical restraints of the compressed mat are provided onone side by the substantially rigid foraminous anode against which theflexible membrane bears and on the other side by a substantially rigidforaminous pressure plate which may itself be the active catalytic layerof the cathode or it may be the current distributor against which theforaminous catalytic layer of the cathode bears.

In the latter case the resiliently compressed mat has two functions; oneis to provide and secure a certain separation, preferably from 1 to 4mm, between the surface of the membrane and the surface of the activecathode layer during operation of the cell, the other is to press theactive cathode layer against the rigid current distributing means for asatisfactory operation of the cathode.

Considering that the active cathode layer most advantageously is made ofa catholyte resistant metal such as iron, stainless steel, nickel,copper or alloys thereof, coated with a catalytic material having a lowhydrogen overvoltage, such as a noble metal (Pt, Rh, Ru, Ir, Pd) oralloy thereof or a conductive oxide thereof or of other metals and thatthese coatings, which impart low hydrogen overvoltage characteristic tothe main cathode layer, are seldom permanent but need to be renewedafter a certain period of operation, it is evident that a greatadvantage derives from the possibility, offered by this preferredembodiment of the invention, to substitute the worn out active cathodelayer without having to disconnect or to cut welds and to weld orconnect back in place the newly coated cathode.

In fact, in the cell of the present invention, the active cathode layermay be a thin foraminous coated metal screen which is simply sandwichedbetween the resiliently compressed spacer layer or mat and asubstantially rigid pressure plate or a series of spaced ribs or stubs,acting as the current distribution means to the active cathode layer.

The resiliently compressible mat, forming the spacer layer of relativelyhigh hydrogen overvoltage, is pliable and springlike in character andwhile capable of being compressible to a reduction of up to 60 percentor more of its uncompressed thickness against the membrane byapplication of pressure from the compression means, it is also capableof springing back substantially to its initial thickness upon release ofthe clamping pressure. Thus, by its elastic reaction memory, it appliesand maintains substantially uniform pressure against the membrane sinceit is capable of distributing pressure stress and of compensating forirregularities in the surfaces with which it is in contact. It isflexible enough to bend in all directions and to assume the contours ofthe membrane. The compressible mat should also provide ready circulationof the electrolyte to and from the membrane surface.

Thus, the compressible layer is open in structure and includes a largefree volume. The resiliently compressible mat is essentiallyelectrically conductive on its surface, generally being made of a metalresistant to the electrochemical attack of the electrolyte in contacttherewith and it thus helps distributing polarity and current over themain active electrode layer.

A preferred embodiment of the resilient spacer layer of the presentinvention is characterized in that it consists of a substantially openmesh, planar, electroconductive metal-wire article or screen having anopen network and is comprised of wire or fabric resistant to theelectrolyte and the electrolysis products and in that some or all of thewires form a series of coils, waves or crimps or other undulatingcontour whose diameter or amplitude is substantially in excess of thewire thickness and preferably corresponds to the article thickness,along at least one direction parallel to the plane of the article. Ofcourse such crimps or wrinkles are disposed in the direction across thethickness of the screen.

These wrinkles in the form of crimps, coils, waves or the like have sideportions which are sloped or curved with respect to the axis normal tothe thickness of the wrinkled fabric so that, when the layer iscompressed, some displacement and pressure is transmitted laterally soas to make distribution of pressure more uniform over the electrode areaor surface. Some coils or wire loops which, because of irregularities onthe planarity or parallelism of the surface compressing the fabric, maybe subjected to a compressive force greater than that acting on adjacentareas, are capable of yielding more to discharge the excess force bytransmitting it to the neighboring coils or wire loops. Therefore, thefabric is effective in acting as a pressure equalizer to a substantialextent and in preventing the elastic reaction force acting on a singlecontact point to exceed the limit whereby the membrane is excessivelypinched or pierced. Of course, such self-adjusting capabilities of theresilient layer are also instrumental in obtaining a good and uniformcontact distribution over the entire surface of the electrode.

One very effective embodiment desirably consists of a series ofhelicoidal cylindrical spirals of wire whose coils are mutually woundwith the ones of the adjacent spiral in an intermeshed or interloopedrelationship. The diameter of the spirals is 5 to 10 or more times thediameter of the wire of the spirals. According to this preferredarrangement, the wire helix itself represents a very small portion ofthe volume enclosed by the helix and therefore the helix is open on allsides thereby providing an interior channel to permit circulation of theelectrolyte.

It is not, however, necessary for the helicoidal cylindrical spirals tobe wound in an intermeshed relationship with the adjacent spirals asprevious described, and they may also consist of single adjacent metalwire spirals. In this case, the spirals are juxtaposed one besideanother with the respective coils being merely engaged in an alternatesequence.

According to a further embodiment, the spacer layer consists of acrimped knitted mesh or fabric of metal wire wherein every single wireforms a series of waves of an amplitude corresponding to the maximumheight of the crimping of the knitted mesh or fabric. As an alternative,two or more knitted meshes or fabrics, after being individually crimpedby forming may be superimposed one upon another to obtain a layer of thedesired thickness.

The crimping of the metal mesh or fabric imparts to the layer a greatcompressibility and an outstanding resiliency to compression under aload which may be at least about 50-2000 grams per square centimeter(g/cm²) of surface applying the pressure.

The mat is capable of being compressed to a much lower thickness andvolume. For example, it may be compressed to about 50 to 90 percent oreven lesser percent of its initial volume and/or thickness and is,therefore, pressed or compressed between the membrane and the activecathode layer.

The mat is moveable or slideable with respect to the adjacent surfacesof the membrane and of the active cathode layer between which it iscompressed. When clamping pressure is applied, the wire loops or coilsconstituting the resilient mat may deflect and slide laterally anddistribute pressure uniformly over the entire surfaces with which itcontacts.

A large portion of the clamping pressure of the cell is elasticallymemorized by every single coil or wave of the metal wires forming thespacer layer.

Preferably, the resilient mat is compressed to about 80 to 30 percent ofits original uncompressed thickness under a compression pressure whichis comprised between 50 and 2000 grams per square centimeter ofprojected area. Even in its compressed state, the resilient mat must behighly porous and the ratio between the voids volume and the apparentvolume of the compressed mat expressed in percentage is advantageouslyat least 75% (rarely below 50%) and preferably is comprised between 85%and 96%.

The diameter of the wire utilized may vary within a wide range dependingon the type of forming or texturing being low enough in any event toobtain the desired characteristics of resiliency and deformation at thecell-assembly pressure. An assembly pressure corresponding to a loadbetween 50 and 500 g/cm² of electrodic surface is normally required toobtain a good electrical contact between the active cathode layer andthe cooperating current distribution structures or collectors althoughhigher pressures may be used.

It has been found that by providing a deformation of the resilientspacer layer of the invention of about 1.5 to 3 millimeters (mm) whichcorresponds to a compression not greater than 60% of the thickness ofthe non-compressed article, at a pressure of about 400 g/m² of projectedsurface a contact pressure at the active cathode layer may be obtainedwithin the above cited limits also in cells with a high surfacedevelopment and with deviations from planarity up to 2 millimeters permeter (mm/m).

The metal wire diameter is preferably between 0.1 or even less and 0.3millimeters, while the thickness of the non-compressed article, that is,either the coils' diameter or the amplitude of the crimping is 5 or moretimes the wire diameter, preferably in the range of 4 to 10 millimeters.Thus it will be apparent that the compressible section encloses a largefree volume, i.e. the porportion of occupied volume which is free andopen to electrolyte flow and gas flow.

In the wrinkled (which includes these compressing wire helixes) fabricsdescribed above, this percent of free volume is about 75% of the totalvolume occupied by the fabric and this percent of free volume rarelyshould be less than 25% and preferably should not be less than 50%.Pressure drop in the flow of gas and electrolyte through such a fabricis negligible.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention herein contemplated may be applied to an electrolytic cellsuch as the one diagrammatically illustrated in the accompanyingdrawings in which:

FIG. 1 is a diagrammatic horizontal sectional view of the cell havingthe double layer electrode installed therein, and

FIG. 2 is a diagrammatic vertical sectional view of the cell of FIG. 1.

As shown, the cell comprises an anode end plate 1 and a cathode endplate 2, both mounted in a vertical plane with each end-plate in theform of a channel having side walls respectively enclosing an anodespace 3 and a cathode space 4. Each end plate also has a peripheral sealsurface on side-walls projecting on each side of the cell from the planeof the respective end plate, 5 being the anode seal surface and 6, beingthe cathode seal surface. These surfaces, with the interposition ofsuitable gaskets, not shown in the drawing, bear against a membrane ordiaphragm 7, which stretches across the enclosed space between the sidewalls separating anode from cathode.

The anode 8 may comprise a relatively rigid uncompressible sheet ofexpanded titanium metal or other perforate, anodically resistantsubstrate, preferably having a non-passivatable coating thereon such asa metal or oxide or mixed oxide of a platinum group metal. This sheet issized to fit within the side walls of the anode back plate and issupported rather rigidly by spaced electroconductive ribs 9 which arefastened to and project from the web or base of the anodeend-plateplate 1. The spaces between the ribs provide for ready flow ofanolyte which is fed into the bottom and withdrawn from the top of suchspaces. The entire end plate and ribs may be of graphite andalternatively, it may be of titanium clad steel or other suitablematerial. The rib ends bearing against the anode sheet 8 may or not becoated, e.g. with platinum or like metal to improve electrical contactand the anode sheet 8 may, if desired, be welded to the ribs 9. Theanode rigid foraminous sheet 8 is held firmly in an upright position.This sheet may be of expanded metal having upwardly inclining openings10 directed away from the membrane (see FIG. 2) to deflect rising gasbubbles towards the space 9 and away from the membrane.

On the cathode side, ribs 11 extend outward from the base of the cathodeend plate 2 a distance which is a fraction of the entire depth of thecathode space 4. These ribs are spaced across the cell to provideparallel space for vertical electrolyte flow from bottom to top andengage the cathode, which is in sheet or layer form. The cathode endplate and ribs may be made of steel or a nickel iron alloy or othercathodically resistant electroconductive material. On the conductiveribs 11 is welded a relatively rigid pressure plate 12, which isperforate and readily allows circulation of electrolyte from one sidethereof to the other. Generally these openings or louvers are inclinedupward and away from the membrane or compressible electrode toward thespace 4 (see also FIG. 2) The pressure plate is electroconductive andserves to impart cathodic polarity to the electrode and to applypressure thereto and it may be made of expanded metal or heavy screen ofsteel, nickel, copper or alloys thereof.

The main or active cathode layer may advantageously be made of a fineflexible screen 13 of a cathodically resistant electroconductivematerial, such as nickel, stainless steel, iron, copper or alloysthereof, coated with a cathodically resistant catalytic material havinga low hydrogen overvoltage. Many catalytic materials for hydrogenevolution in caustic solutions are known in the art, particularlysuitable materials are the noble metals such as platinum, ruthenium,palladium, rhodium, iridium and osmium, their alloys and oxides, Raneynickel, molybdenum and tungsten alloys. Any of these materials can beused successfully to coat the cathode screen.

The resiliently compressible spacer layer 14, interposed betweenmembrane 7 and the main active layer 13 may be made of a crimpedcorrugated or wrinkled compressible wire-mesh fabric which fabric isadvantageously an open mesh knitted-wire mesh of the type described inU.S. Pat. No. 4,343,690, wherein the wire strands are knitted into arelatively flat fabric with interlocking loops. This fabric is thencrimped or wrinkled into a wave or undulating form with the waves beingclose together, for example 0.3 to 2 centimeters apart, and the overallthickness of the compressible fabric is 2 to 10 millimeters. The crimpsmay be in a zig-zag or herringbone pattern and the mesh of the fabric iscoarser, i.e. has a larger pore size than that of screen 13.

In this preferred embodiment the resiliently compressible space layer 14is instrumental in providing for a good electrical contact between thepressure plate 12 and the main or active cathode layer 13, which ispressed by the spacer layer 14 against the current distributor pressureplate 12 uniformly over the entire electrode surface.

The resiliently compressible spacer layer 14, also presses the maintainsthe flexible membrane 7 bearing against the rigid foraminous anode 8,thus preventing its movement and fluttering in the cell.

The layer 14 effectively spaces the surface of the main or activecathode layer from the membrane of an easily predetermined distancewhich preferably may be comprised between 1 and 4 mm.

Since the spacer layer 14 has a higher hydrogen overvoltage than theactive layer 13, the electrode reactions take place substantially at thesurface of the catalytic screen 13 and because of the very openstructure of the compressed layer 14 of fine metal wire.

The products of the electrode reaction are easily diluted and quicklyremoved from the surface of the membrane, thus effectively preventinghigh concentration gradients across the surface of the membrane.

In the operation of this embodiment, substantially saturated sodiumchloride aqueous solution is fed into the bottom of the anolytecompartment of the cell and flows upward through channels or spaces 3between ribs 9 and depleted brine and evolved chlorine escapes from thetop of the cell. Water or dilute sodium hydroxide is fed into the bottomof the cathode chamber and rises through channels 4 as well as throughthe voids of the compressed spacer layer 14 and evolved hydrogen andalkali is withdrawn from the top of the cell.

Electrolysis is caused by imparting a direct current electric potentialbetween the anode and cathode end plates.

As shown in FIG. 2, the openings in pressure plate 12 are louvered toprovide an inclined outlet directed upwardly away from the compressedfabric layer 14, whereby some portion of evolved hydrogen and/orelectrolyte escapes to the rear electrolyte chamber 4. Therefore, thevertical spaces at the back of the pressure plate 12 and the spaceoccupied by the compressed fabric 14 are provided for upward catholyteand gas flow.

According to the improved method of this invention for the electrolysisof sodium chloride, aqueous brine containing from 140 to 300 grams perliter of sodium chloride is circulated within the anode compartment ofthe cell. Chlorine is evolved at the anode, while the solvated ions tendto migrate through the cation membrane and reach the cathode wherecaustic soda of substantial concentration above 15-20% by weight andhydrogen is evolved. Solutions containing 25 to 40% by weight of alkalimetal hydroxide may be produced with anode and cathode efficienciesabove 90%, frequently above 94%.

The following examples are illustrative:

EXAMPLE

A laboratory size electrolytic cell was manufactured having an effectiveelectrode area 100 millimeters (mm) high and 100 millimeters (mm) wide.

The cell frames and back plates were made of titanium for the anodicportion and of stainless steel (AISI 316) for the cathodic portion.

The anode was an expanded titanium sheet 1.5 mm thick, coated with a nonpassivatable catalytic coating of a mixture of oxides of Ruthenium andTitanium in the respective weight ratio of 1 to 1, as referred to themetals, obtained by thermal decomposition of a solution of the salts ofthe metals.

The depth of the anode chamber behind the anode was 12 millimeters (mm).

The membrane was a laminated sheet having a thickness of about 0.25 mm,comprising two layers of cation exchange resin laminated together withan interlayer of a polytetrafluoroethylene screen, as mechanicalsupport. The two layers are made of a copolymer of tetrafluoroethyleneand a perfluorovinylether, one containing sulphonic groups and the othercontaining carboxylic groups.

The membrane was assembled in the cell with its carboxylic layer facingthe catholyte compartment.

The cathode structure comprised:

(a) a current collector in the form of a perforated sheet of AISI 316,2.0 millimeters thick, provided with holes of diameter 3.0 mm, with apitch of 5 mm, welded on

AISI 316 vertical ribs. The depth of the cathode chamber behind thecurrent collector screen was 18 millimeters.

(b) a main or catalytic cathode layer in the form of a 25 mesh nickelscreen coated with a 7 to 8 gram per square meter loading of an alloy ofruthenium (80 to 85 percent) and nickel (15 to 20 percent), providingfor an exceptionally low hydrogen overvoltage.

(c) a resiliently compressed spacer layer in the form of a mat made ofthree double layers of loosely knitted nickel wire of a diameter of 0.11millimeters.

The catalytic cathode layer (b) was interposed between the rigid currentcollector (a) and the resilient spacer layer (c) and, upon the clampingtogether of the cell, the current collector was compressing theresilient mat against the surface of the membrane, which membrane wasbearing in turn against the rigid anode. The compression correspondingto a pressure of about 400 grams per square centimeter was reducing thethickness of the resilient mat, interposed between the active cathodescreen and the membrane, from its original uncompressed thickness ofabout 6 mm down to about 2.7 millimeters. Therefore, the distancebetween the surface of the anode and the surface of the active cathodelayer was about 2.7 millimeters plus the thickness of the membrane, thatis practically it was comprised between 2.7 and 2.8 millimeters.

The cell operated at the following conditions:

Current density: 300 A/m²

Anolyte concentration: 175 g/l of NaCl

Catholyte concentration: 30% by wt. of NaOH

Temperature: 90° C.±1° C.

Cell voltage: 3.12 V±0.02

Cathodic current efficiency: 94.5%

Oxygen in chlorine gas: 0.1% by volume.

REFERENCE EXAMPLE

The same cell described in Example 1 was disassembled and the main (orcatalytic) cathode screen of coated nickel (b) was placed against thesurface of the membrane, the resilient mat of knitted nickel wire (c)was placed between the rigid current collector (a) and the activecathode screen.

Upon re-assembly of the cell, the resilient mat was compressed down to athickness of about 2.7 mm, thereby pressing the active cathode screenagainst the surface of the membrane. Therefore, the distance between thesurface of the anode and the surface of the cathode corresponded to thethickness of the membrane, that is about 0.25 millimeters.

The cell was operated at exactly the same conditions as indicated in theprevious example and the results were as follows:

Cell voltage: 3.19 V±0.02

Cathodic current efficiency: 93%

Oxygen is chlorine gas: 0.5% by volume

The method of the invention may be practiced with any type of ionpermeable membrane.

The membrane may be of the monolayer type or it may be a laminatedmembrane comprising different layers made of different ion exchangeresins and the membrane may also include reinforcing fibers or fabrics.

The surfaces of the membrane may be modified either in their chemicalcomposition or in their physical morphology, for example the membranemay have a roughened surface.

Also the membrane may have a porous layer of resin or of particulatematerial forming a microporous layer over the surface of the membrane,said layer being either conductive or non conductive in character.

As it will be obvious to the expert, the current distribution meanswhich in the preferred embodiment described in the accompanying drawingsare depicted in the form which comprises a substantially rigidforaminous plate 12, may be of different nature, for example the activecathode screen 13 may be pressed by the resilient wire mat directlyagainst the vertical ribs 11, extending from the cathode end plate.

Preferably in the latter case the active cathode screen 13 can be madeof a heavier gauge screen and the distribution of the vertical ribs maybe made more dense, that is with a larger number of ribs per unit ofwidth of the cell compartment, in order to provide sufficient number ofelectric contact between the active screen and the current distributionmeans.

What I claim is:
 1. A method of generating chlorine which compriseselectrolyzing an aqueous alkali metal chloride in a cell having an ionpermeable membrane dividing the cell into an anode compartment, an anodein said anode compartment, and a cathode compartment, a cathode in thecathode compartment, said cathode comprising a screen having a lowhydrogen overvoltage in contact with substantially rigid currentdistribution means and a high hydrogen overvoltage resilientlycompressible wire mat compressed between the membrane and the lowhydrogen overvoltage screen to maintain the low hydrogen overvoltagescreen spaced from the membrane and wherein the mat presses the lowvoltage screen against the current distribution means.
 2. The method ofclaim 1 wherein the membrane is compressed by said resilientlycompressible wire mat against the surface of the anode.
 3. The method ofclaim 1 wherein said membrane is an ion exchange polymer.
 4. The methodof claim 1 wherein said screen has 3 to 10 mesh openings per centimeter.5. The method of claim 1 wherein said wire mat comprises a surface of amaterial selected from the group of silver, stainless steel, and nickel.6. The method of claim 1 wherein the separation between the surface ofthe membrane and the surface of the cathode is 1-4 mm.
 7. The method ofclaim 1 wherein said screen comprises a metal selected from the group ofiron, stainless steel, nickel, and copper, and alloys thereof; and beingcoated with a noble metal, alloy thereof, conductive oxide thereof,Raney nickel, or molybdenum and tungsten alloys.
 8. The method of claim1 wherein said wire mat comprises a series of helicoidal cylindricalspirals of wire whereby the diameter of the spirals is 5 to 10 times thediameter of the wire.
 9. An electrolytic diaphgram cell comprising aion-exchange membrane dividing the cell into an anode compartment and acathode compartment, a foraminous anode in the anode compartment, aforaminous cathode in the cathode compartment, characterized in thatsaid cathode comprises a screen having a surface of low hydrogenovervoltage spaced from the surface of the membrane by a resilientlycompressed wire mat having a surface of higher hydrogen overvoltage thanthe surface of said screen and said screen being pressed by saidresiliently compressed wire mat against current distribution meansrigidly mounted in the cathode compartment.
 10. The cell of claim 9wherein the membrane bears directly against the surface of theforaminous anode.
 11. The process of claim 1 wherein said mat is capableof springing back substantially to its initial thickness and whereinsaid mat applies and maintains substantially uniform pressure againstthe membrane and is sufficiently flexible so as to bend in alldirections and to provide ready circulation of the electrolyte to andfrom the membrane surface.
 12. The cell of claim 9 wherein said membraneis an ion exchange polymer.
 13. The cell of claim 9 wherein said screenhas 3 to 10 mesh openings per centimeter.
 14. The cell of claim 9wherein said wire mat comprises a surface of a material selected fromthe group of silver, stainless steel, and nickel.
 15. The cell of claim9 wherein the separation between the surface of the membrane and thesurface of the cathode is 1-4 mm.
 16. The cell of claim 9 wherein saidscreen comprises a metal selected from the group of iron, stainlesssteel, nickel, and copper, and alloys thereof; and being coated with anoble metal, alloy thereof, conductive oxide thereof, Raney nickel, ormolybdenum and tungsten alloys.
 17. The cell of claim 9 wherein saidwire mat comprises a series of helicoidal cylindrical spirals of wirewhereby the diameter of the spirals is 5 to 10 times the diameter of thewire.
 18. The cell of claim 3 wherein said mat is capable of springingback substantially to its initial thickness and wherein said mat appliesand maintains substantially uniform pressure against the membrane and issufficiently flexible so as to bend in all directions and to provideready circulation of the electrolyte to and from the membrane surface.19. The process of claim 17 wherein said mat is compressed to about 80to 30 percent of its original uncompressed thickness under a compressionpressure of between 50 and 2000 grams per square centimeter of projectedarea and the ratio between voids volume and apparent volume of thecompressed mat is at least 75%.
 20. The process of claim 18 wherein saidmat is compressed to about 80 to 30 percent of its original uncompressedthickness under a compression pressure of between 50 and 2000 grams persquare centimeter of projected area and the ratio between voids volumeand apparent volume of the compressed mat is at least 75%.